1. Field of the Invention
The present invention is broadly concerned with novel, crosslinkable, cellulose compositions useful for forming films for use in polarizing plates and other optical devices.
2. Description of the Prior Art
Cellulose ester films have been used for a variety of optical applications. More recently, cellulose esters have found use in liquid crystal displays due to their unique combination of transparency, good surface smoothness, optical isotropy, and optimal moisture vapor transmission rate (MVTR). Cellulose esters have found utility as protective films for polarizing plates, optical compensation films (phase retarders), substrate films for other functional films, and as various other functional films (e.g., anti-reflective films for plasma displays, films for organic electroluminescent displays).
For many years, solvent-cast cellulose triacetate films have been used as a photographic film support due to the tough, flame retardant nature of these materials. Additionally, triacetate films are widely used as protective layers of polarizer elements for LCD applications where its physical characteristics and the dimensional uniformity and surface quality imparted by solvent casting have made cellulose triacetate the first choice for many optical films.
Despite the excellent optical properties of solvent-cast, cellulose ester films, environmental concerns about solvents conventionally used in the casting of the films have created a need for new methods of manufacturing the films, or for a new kind of film support. It has been reported in the art that cellulose triacetate cannot be melt-cast because its melting point is above its decomposition temperature. As for solvent casting of cellulose triacetate, few solvents suitable for industrial use have been found that are more acceptable than the conventional ones, which are generally toxic and environmentally unfriendly.
One possible way to completely eliminate the need for solvents is to melt cast a thermally stable polymer such as poly(ethylene terephthalate). Indeed, this type of polymer is used commercially for the manufacture of supports for photographic sheet films such as x-ray films and graphic arts films. It is not suitable, however, for many kinds of optical films, including roll films for amateur cameras. In this use the polyester film develops curl or “core set” when wound on the film spool. Cellulose triacetate also develops curl when wound (and a certain amount of core set is desirable), but when the cellulosic film is exposed to moisture the curl of the hydrophilic cellulosic film is relaxed and the film lies flat. Poly(ethylene terephthalate) films, on the other hand, do not relax their core set with simple humidity, so they are unsatisfactory for photographic roll films. Other polymers lack one or more of the combination of properties and capabilities that make solvent cast cellulose triacetate successful as a preferred optical film relative to melt cast films.
Esters of cellulose hydroxyl groups have been made over a wide range with both single and mixed acids for different uses. In a cellulose ester, the extent of esterification with a given substituent is described as the degree of substitution (DS), in which the maximum substitution per anhydroglucose unit is typically three. Cellulose diacetate (DSac=2.45), unlike the triacetate, has a sufficiently low melting point that, with adequate plasticizer addition, it can be melt extruded, thus avoiding the need for toxic and environmentally unfriendly solvents. Mixed esters, or replacement of acetyl groups of the triacetate with appropriate levels of propionic, butyric, or other higher ester groups can accomplish the same purpose. Films made from these known cellulose ester compositions of lower acetyl content are generally deficient in properties that are necessary for photographic roll film supports, most notably in stiffness and heat distortion temperature.
Additionally, in recent years, there has been a drive for thinner, lighter, highly transparent optical films with improved heat resistance, moisture resistance, chemical resistance, dimensional stability, and mechanical strength. As films become thinner, a wide range of issues are encountered. For example, films may become less uniform in thickness, the surface may become mottled, ultraviolet (UV) light resistance may decrease, the MVTR may increase, and dimensional stability may suffer.
With respect to protective films for polarizer plates, as the film thickness is reduced, the MVTR increases, which results in less durable polarizing plates, especially under high temperature, high humidity environments. It is known that increased plasticizer content can decrease the MVTR, but elevated plasticizer levels reduce the glass transition temperature (Tg) of the cellulose ester films, which is associated with deterioration of dimensional stability of the film. In addition, it has been shown that an increase in the loading of plasticizer can lead to exudation of the plasticizer to the surface of the cellulose ester films, which can result in inhomogeneous plasticizer distribution or contamination of the film web or rolls.
Cellulose esters used as protective films for polarizer plates also may contain UV absorbers to protect the polarizing element from UV light. As protective films become thinner, the films cannot shield a sufficient amount of UV light. Consequently, additional UV absorber is required, which may exude out of the cellulose ester and contaminate the film web or rolls, or may lead to increased haze in the finished film. A thin protective film for polarizer plates with excellent MVTR, excellent film performance and dimensional stability, good plasticizer retention, and good UV absorber retention is required.
Unfortunately, cellulose esters are also used as optical compensation films for LCDs. These films may be manufactured by coating a liquid crystalline compound-containing solution on an anisotropic cellulose ester film. As in the case of the protective films for polarizing plates, UV absorbers are added to the compensation films. Exudation of the UV absorber in compensation films may lead to haze in the film, or the exudates may contaminate the liquid crystalline compound, leading to disordering of the liquid crystalline compounds. An optical compensation film for polarizer plates with excellent dimensional stability, excellent film performance, and good UV absorber retention is needed.
Thin cellulose ester films typically suffer from increased MVTR and reduced dimensional stability. In addition, cellulose ester films may exude plasticizers and/or UV absorbers under certain conditions, including reduced film thickness, elevated temperature, and elevated humidity. Attempts have been made to address these issues through addition of alternative compounds to the cellulose ester compositions, through application of hard coats to cellulose ester films, or through use of alternative materials to cellulose esters.